The present invention relates to an apparatus for applying coating liquid to a magnetic tape or the like.
One process of applying coating liquid to a magnetic tape is described below as an example. A gravure coating method is a representative example of a technique for uniformly applying coating liquid to a long web as thin as approximately tens of micrometers in a thickness not more than tens of micrometers. According to the gravure coating method, numerous minute concave grooves are formed on the surface of a rotary roll. With the rotation of the rotary roll, a volume of coating material is measured and supplied to the concave grooves so as to continuously transfer the coating material to the web. The coating material is required to coat the magnetic tape in a uniform thickness and the surface of a film formed on the tape is required to be smooth. The maximum coating speed is as slow as 200 m/min to 250 m/min because there is a limit to the speed of the roll which is the speed at which a droplet will be separated from the surface of the rotary roll by centrifugal force. With rationalized mass production in recent years, there is a growing demand for a coating method, for example, capable of reliably carrying out coating and at a speed as fast as not less than 400 /min.
As disclosed in Japanese Laid-Open Patent Publication No. 63-153194, the inventors of the present invention proposed a coating method and an apparatus therefor. The coating apparatus comprises a die for applying a coating solution diluted with a solvent in stripes to a thin web, for example, a magnetic tape in the travel direction, or the longitudinal direction thereof. According to this invention, the edge of the web as well as the entire surface thereof can be favorably coated with the coating solution.
However, the above prior art has some problems in continuously forming a film in a uniform thickness on a magnetic tape or the like if the tape is as long as not less than tens of thousands of meters.
It takes much time and labor for the above prior art to coat the web for the following reason. That is, in the process of carrying out a continuous coating for a long time, coating liquid diluted with a solvent having a high volatility such as methylene group or ethylene group coagulates in the periphery of a discharge opening 15 and the edge surface of a groove 14 as shown by oblique lines in FIG. 7. As a result, the configuration and size of the flow path of the coating material varies as compared with those of the groove 14. When the coating liquid coagulates on a part of a guide 13, the performance of the sliding contact between the web 30 and the guide 13 deteriorates. Consequently, while the web is traveling, it does not locally contact the guide 13. As a result, a film of a uniform thickness cannot be formed on the web 30. In order to overcome such a disadvantage, it is necessary to wash an upper nozzle 12 of the die and a lower nozzle 18 thereof with a solvent after only a short period of use to remove a coagulated substance. In addition, in resuming the coating operation after suspending the operation and replacing the web 30, it is necessary to disassemble the main body 11 and wash out substances which have collected in the discharge opening 15 of the upper nozzle 12 or the groove 21.